A technology has been proposed for controlling a vehicle including a fuel cell system while the fuel cell system is being started and rated operation cannot be performed. For example, Japanese Patent Application Publication No. JP 2003-249251 A, Japanese Patent Application Publication No. JP 2004-55379 A, Japanese Patent Application Publication No. JP 2002-313391 A disclose technologies in each of which when a temperature of a fuel cell is low, the fuel cell is heated by a heater so as to be smoothly started. Japanese Patent Application Publication No. JP 2000-315511 A discloses a technology in which when a driver depresses an accelerator pedal to a large extent, supply of electric power to a fuel reforming system is limited, and the electric power is used for driving a vehicle.
However, no consideration has been given to improvement of driveability which requires a driving system of a vehicle to respond as intended by a driver. For example, in the technology disclosed in the Japanese Patent Application Publication No. 2000-315511 A, an amount of electric power that can be used in a driving system of the vehicle can be increased. However, since the amount of the electric power that can be used in the driving system is not increased in quick response, the increase in the amount of the electric power that can be used in the driving system does not contribute to improvement of the drivability. On the contrary, the drivability may be decreased by the increase in the amount of the electric power that can be used in the driving system. Since the electric power is not increased in quick response, an output of the driving system may be increased after a certain delay, and accordingly unexpected acceleration may occur. Further, not only vehicles including a fuel cell system, but also a wide range of electric power sources including a fuel cell system have this problem concerning operability.